This invention relates generally to positive displacement, variable delivery pumping apparatus of the vane pump-type driven by either a constant speed or a varying speed power source and delivering controlled variable flow of a pumped liquid, such as fuel. More particularly, the invention relates to an improved two stage vane pump apparatus for delivering fuel to an aircraft jet engine or gas turbine at a controllable fuel flow rate which is independent of the driven speed of the pump.
Typically, liquid fuel is pumped to a gas turbine, such as a jet aircraft engine, using positive displacement, variable delivery pumping apparatus. Such positive displacement pumps have included gear pumps, vane pumps and piston pumps. Variable delivery has been achieved by changing the geometry of the positive displacement pump or by bypassing liquid to the inlet, both of which decrease the overall efficiency and require power as the flow is varied.
As gas turbine engine technology advances, engine manufacturers are requiring fuel systems which are light, compact, very efficient, and preferably operate from a single fuel demand control signal without regard for speed of the engine, and suitable for driving by any power source, i.e., the engine itself or a dedicated motor. For example the fuel needs of an aircraft jet engine are quite different during takeoff and during high altitude cruising.
Two stage pumping arrangements for jet engine fuel are also known which are combined with various accessory apparatus to meet the particular requirements of aircraft jet engines, which involve a large number of hydromechanical accessory devices in the overall fuel control system. All of the above add to the cost. It would be desirable to have a simplified positive displacement, variable delivery pumping apparatus which provides for variable flow of the pumped liquid suitable for either constant speed or variable speed drive and without the need to change the geometry of the pump hardware. Furthermore it would be desirable to have an improved variable delivery, positive displacement pumping apparatus which will operate without bypassing flow back to the inlet using simplified controls. Accordingly, one object of the present invention is to provide an improved positive displacement, variable delivery pumping apparatus which provides variable flow with simplified controls and more efficiently than existing pumping apparatus.
Another object of the invention is to provide an improved fuel delivery system for an aircraft jet engine.
Still another object of the invention is to provide a variable flow pump suitable for constant or variable input drive speed which is efficient and which operates with a simple control system.